Let's start with some relevant anatomy.
The choroid plexus is made up of ependymal cells which produce cerebrospinal fluid - a fluid that helps provide buoyancy and protection, as well as metabolic fuel for the brain.
Highest up, are two C-shaped lateral ventricles that lie deep in each cerebral hemisphere.
The two lateral ventricles drain their cerebrospinal fluid into the third ventricle, which is a narrow, funnel-shaped, cavity at the center of the brain.
Cerebral aqueduct stenosis develops when there’s a blockage of the cerebral aqueduct between the third and fourth ventricle, and most of the time this blockage is caused by a tumor.